EMongoDataProvider does not work if sort is not specified

EMongoDataProvider is broken now if you do not specify a sort order (commit 75d674c seems to be responsible).

In EMongoDataProvider class, on line 103, EMongoSort#getOrderBy is passing its result to EMongoCursor#sort (which expects an array). Problem is that EMongoSort#getOrderBy:73 would return the default order as string, which causes an error to be raised.